In desktop computing, it is now common to use more than one display device such as a monitor, television screen or even a projector. Traditionally, a user would have a computing device with a single display device attached, but now it is possible to have more than one display device attached to the computer, which increases the usable area for the worker. For example, International Patent Application Publication WO 20071020408 discloses a display system which comprises a plurality of display devices, each displaying respectively an image; a data processing device connected to each display device and controlling the image displayed by each display device; and a user interface device connected to the data processing device. Connecting multiple display devices to a computer is a proven method for improving productivity. A computer supplying display data to one or more display devices is commonly called a host.
After the display data has been generated by the host, it is sent to one of a plurality of frame buffers. In general, a different frame buffer is used for each different type of display data, for example, image data and video overlay data. The display data from each of these frame buffers is fetched by a dedicated video display controller which performs blending of the different types of display data and, optionally, mapping and/or transformational functions before supplying the processed display data to the display device. The mapping functions may include dithering, which is the application of noise to randomise errors caused by processing in order to minimise the appearance of visual artefacts such as banding, and scaling of the image to be displayed so that it fits appropriately on the connected display device. Transformational functions, such as flipping, mirroring, rotating or other transformations may also be applied where image data is being supplied to the display device via an unusual input or the display device is in an unusual orientation. The video display controller may optionally also perform compression.
Conventionally, a video display controller is arranged in hardware such that there is one video display controller per display device output. This limits the number of display devices that can be connected to a host. On the other hand, providing multiple video display controllers in order to allow multiple display devices to be connected, leads to wasted resources as they may not all be used, and even if all the video display controllers are connected to display devices, it is unlikely that they will all be used to their full capacity.
Thus, it is desirable to overcome, or at least reduce, the disadvantages of the prior art noted above.